Electroplating a black nickel-zinc alloy deposit

ABSTRACT

BLACK ELECTROPLATES ARE OBTAINED FROM AN ALKALINE SOLUTION OF NICKEL, ZINC, AND POLYETHYLENEIMINE.

United States Patent Office 3,681,21 l Patented Aug. 1, 1972 rm. 01.C231) 5/32 US. Cl. 204-43 7 Claims ABSTRACT OF THE DISCLOSURE Blackelectroplates are obtained from an alkaline solution of nickel, zinc,and polyethyleneimine.

BACKGROUND OF THE INVENTION Articles having lustrous black finishes aredesirable and suitable for numerous applications and uses, particularlyfor decorative purposes and/or to promote absorption or radiation ofheat. Matte black finishes are also important in providingnon-reflecting surfaces in industrial and military instruments ofvarious types such as the protoscope and the naval anemometer. However,black deposits produced heretofore have been found unsatisfactorybecause of poor abrasion resistance and/or poor adherence of the nickeldeposit to the substrate surfaces and/or poor ductility characteristics.

The prior art black nickel plating processes, in addition to yieldingfilms characterized by unsatisfactory physical properties, have beenfound diflicult to control so as to produce a uniform black coating. Ithas been recognized that factors contributing to difficultiesencountered in the controllabilty of known black plating processes oftenstem from the comparatively rigid and restrictive operating conditionsimposed by those processes, particularly the pH control, range ofconcentration of the constituents of the plating baths, current densitylimits, plating times, and the like. For example, the prior artsolutions are operated under acid conditions and the normal operatingrange for the pH of the well known black nickel sulfate type platingbaths generally falls within the relatively narrow range of 5 to 6. Thepermissible current densities employed in many of these sulfate typeplating baths are often of a narrow and restrictive scope, e.g., about0.5 to about 1.5 amperes per square foot. By using a plating bathcontaining nickel chloride hexahydrate, ammonium chloride, sodiumthiocyanate and zinc chloride, it has been possible to expand thepermissible pH to 2.3 to 5.5, and to employ current densities of 1.6 to6 amperes per square foot. See, e.g., United States Patent 2,844,530.However, this special system is still operated under acid conditionsrequiring special equipment to prevent corrosive acid action and alsorequires plating times that range from 7 /2 to 30 minutes to obtain anacceptable black finish.

It has previously been known to add polyethyleneimine to an acidsolution of a cell feed composition for the electrowinning of copper orzinc (United States Patent 2,853,444). It is also known to employ acombination of an amine oxide and polyethyleneimine in a zinc cyanideelectroplating bath as a brightening agent (United States Patent3,296,105). Surprisingly, it has now been found the polyethyleneiminecan be employed in an alkaline plating solution of nickel and zinc toproduce a black deposit. A much broader range of current densities thanemployed in the prior art can be used and a uniform deposit will beproduced in one to three minutes. Moreover, because an alkaline systemis employed, the corrosive acid conditions of the prior art do not haveto be considered in the equipment requirements. Additionally, the toxicarsenic and cyanide compounds employed in many prior art black nickelformulations are not necessary in the present plating solution.

It is the object of this invention to provide a novel alkaline platingbath from which a durable, uniform, black electroplate can be obtainedin a short period of time using a broad range of current densities. Thisand other objects will become apparent to those skilled in the art fromthe following detailed description.

SUMMARY OF THE INVENTION This invention relates to a novel bathcomposition and process for producing decorative surfaces on conductivesubstrates and more particularly, to a novel electroplating bathsolution and process for the electrodeposition of black finishes onconductive surfaces and to the production of articles having blackfinishes and/or antique finishes.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In accordance with the presentinvention, black electroplates are obtained from an aqueous, alkalinesolution of nickel, zinc, and polyethyleneimine. In preparing theplating bath, the nickel and zinc can be obtained from any suitablemetal containing salt such as the sulfates, chlorides, acetates,carbonates, hydroxides, and the like. Typical metal salts which can beemployed include nickel sulfate, nickel chloride, nickel acetate, nickelcarbonate, nickel hydroxide, zinc sulfate, zinc chloride, zinc acetate,zinc carbonate, zinc hydroxide, and the like salts. Nickel is the majormetal ion constituent of the plating solution. Generally, theconcentration of the nickel ion can be about 2.5 to 50 grams per liter,preferably about 5 to 10 grams per liter. The zinc ions generallyconstitute about 0.25 to 5 grams per liter and preferably are about 0.5to 1.0 gram per liter.

The plating solution can be made alkaline =by employing a suitablehydroxide in an amount sufiicient to provide a pH of about 8 to 14 andpreferably about 10 to 13. The alkali metal hydroxides such as sodiumhydroxide and potassium hydroxide are preferred.

Aqueous solutions containing nickel and zinc ions which are madealkaline with caustic or potassium hydroxide precipitate insolublenickel hydroxide and zinc hydroxide. By adding polyethyleneimine tothese solutions, the precipitated hydroxides are redissolved withoutdecreasing the pH of the aqueous solutions. Without being limited totheory, it is believed that because of the metalpolyethyleneiminecomplexes formed, the electrodeposition potential of the metals,normally very different, are brought closer together and result in analloy codeposit.

The water soluble polyethyleneimines are mildly alkaline, hydrophilic,highly branched polyimines produced by the acid catalyzed polymerizationof ethyleneimine. The polymer is composed of units which have 2 carbonatoms per nitrogen atom and these units are randomly distributed in theapproximate ratios of 1 primary amino nitrogen/2 secondary aminonitrogens/I tertiary amino nitrogen. Commercially availablepolyethyleneimines have molecular weights of about 600 to about 100,000and Brookfield viscosities at 25 C. of about 2.3 to about 1200centipoises when measured in a 5% aqueous solution. Thepolyethyleneimine is employed in an amount sufficient to dissolve anynickel hydroxide or zinc hydroxide which forms. Generally, thepolyethyleneimine will be employed at the rate of about 15 to 250 gramsper liter and preferably at about 30 to 50 grams per liter.

The electroplating solutions of the present invention are prepared bymixing the metal salts, alkaline hydroxide and polyethyleneimine inwater. The article to be plated is used as the cathode and a nickel orcarbon or other inert anode is employed. Direct current is supplied at acurrent density of about 2 to 150 amperes per square foot, preferablyabout 20 to 50 amperes per square foot. The operating temperature of thebath is about 100 to about 180 F., preferably about 150 to 160 F. It hasbeen found that when the bath is operated at the preferred operatingtemperatures and the preferred current densities, a uniform, blackdeposit will be produced in 1 to 3 minutes.

The resulting electroplated article has an adherent black electroplateon the substrate which is generally about to 50 millionths of an inchthick and more usually about 20 to 40 millionths of an inch thick. Thealloy electroplate contains up to 50 weight percent metallic nickel,preferably about 20-30 weight percent, and up to 60 weight percentmetallic zinc, preferably about 45-55 weight percent. Without beinglimited to theory, it is believed that the remainder of the alloyelectroplate contains gaseous hydrogen, occluded polyethyleneimine andpossibly water.

The following examples serve to further illustrate the invention but arenot intended to limit it.

Example I An electroplating solution was prepared by mixing 33 grams ofNiSO -7H O, 15 grams N-aOI-I, 36.5 grams of PEI '6 -(a commercialpolyethyleneimine having a molecular weight of 600 and a Brookfieldviscosity of 2.3 at 25 C. when measured in a 5% aqueous solution), 3grams of ZnSO -7H O, and sufiicient water to make a liter of solution.The pH was 12.5. A brass panel as the cathode and a stainless steelanode were inserted into the solution which was heated to about 160 F.Direct current was applied to the electrodes at a current density of 25amperes per square foot and after 2.5 minutes, a uniform, adherent,black electroplate was produced on the cathode.

Example II Example I was repeated except that the electroplatingsolution contained 30 grams of nickel chloride hexahydrate, 15 grams ofcaustic soda, 32 grams of PEI 6, 1.5 grams of zinc chloride andsufficient water to make up a liter of solution. The pH of the platingbath was 12.5. An adherent, uniform, black electroplate was formed onthe cathode after 2.5 minutes.

Example III Examples I and II were repeated except that PEI 12 (acommercial polyethyleneimine having a molecular weight of 1200 and aBrookfield viscosity of 3.1 at 25 C. when measured in a 5% aqueoussolution) was employed in place of PEI 6. A uniform, adherent, blackelectroplate was obtained.

Example IV The electroplating solution of Example 1 was placed in a 267ml. Hull cell heated to 140 F. and a polished brass cathode was platedfor 4 minutes using a stainless steel anode and a current of 2 ampereson the cathode. The Hull cell is an accepted test method for evaluatingplating performance and characteristics at various current densities. At140 F., the Hull panel showed a uniform black electrodeposit throughoutthe current density range 16-100 amperes per square foot. In the rangeof 2-16 amperes per square foot, the electrodeposit was iridescent.

The temperature was raised to 150 F. and the foregoing Hull test wasrepeated. An adherent, uniform, black electroplate was producedthroughout the range of 6 to 100 amperes per square foot current densitytested.

The temperature was then raised to 160 F. and the foregoing Hull testrepeated to obtain a uniform black 4 electroplate over the range of 2 toamperes per square foot current density tested.

All of the foregoing was repeated except that the electroplatingsolution of Example II was employed and the results obtained wereidentical to the foregoing results.

Example V A cleaned and weighed steel coupon was immersed in theelectroplating solution of Example I at 160 F. The coupon was connectedto the cathode of a DC rectifier and another steel coupon was connectedto the anode of the rectifier. A current of 3.5 amperes (equivalent to acurrent density of 40 amperes per square foot) was passed through thecircuit for 5 minutes. The plated steel cathode was then thoroughlyrinsed in distilled water, dried and reweighed. The deposit wasthereafter removed from the cathode by disolution in a mixture ofhydrogen peroxide and ammonia. Chemical analysis of the resultingsolution showed that the deposit contained 22.5 weight percent nickeland 52.3 percent zinc. No evidence of sulfur was found. It wascalculated that the thickness of the deposit had been 20 millionths ofan inch.

The foregoing procedure was repeated except that a current of 1.8amperes (equivalent to a current density of 20 amperes per square foot)was employed. The analysis of the deposit showed that it contained 25.6weight percent nickel and 46.8 weight percent zinc. It was calculatedthat the thickness of the deposit had been 12.5 millionths of an inch.

Various changes and modifications can be made in the process andproducts hereof without departing from the spirit and scope of theinvention. The various embodiments disclosed herein serve to furtherillustrate the invention but are not intended to limit it.

We claim:

1. A composition for the deposition of a black electroplate whichcomprises an aqueous, alkaline solution of nickel, zinc andpolyethyleneimine wherein the nickel ion concentration is about 2.5 to50 grams per liter, the zinc ion concentration is about 0.25 to 5 gramsper liter, and the polyethyleneimine concentration is about 15 to 250grams per liter.

2. The composition of claim 1 which additionally contains an alkalimetal hydroxide.

3. The composition of claim 2 wherein said alkali metal hydroxide issodium hydroxide.

4. The composition of claim 2 wherein said alkali metal hydroxide ispotassium hydroxide.

5. The composition of claim 1 wherein the pH is about 10 to 13. 6. Aprocess for electrodepositing a black finish on an article whichcomprises subjecting the article to be blackened to the composition ofclaim 1 at a temperature of about 100 to 180 F. and applying a directcurrent at a current density of about 5 to amperes per square foot.

7. The process of claim 6 wherein the temperature is about 150 to F. andthe current density is about 20 to about 50 amperes per square foot.

References Cited UNITED STATES PATENTS 2,419,231 4/1947 Schantz 20443 X3,565,800 2/1971 Wade 20449 X 2,679,475 5 1954 Singler 20449 X 2,844,5307/ 1958 Wesley et al 20443 X 2,989,446 6/1961 Hammond et al 20443 X3,420,754 1/ 1969 Roehl 20443 X GERALD L. KAPLAN, Primary Examiner

